As batteries, particularly rechargeable batteries (e.g., lithium-ion batteries), become increasingly utilized, it is important to monitor the states of these batteries for safety reasons and to ensure their efficient and effective utilization. This can be of particular concern with the use of such rechargeable lithium-ion batteries in vehicles, because of the need for many battery cells connected in series with each other. Lithium-ion batteries are safe, provided certain precautions are met when charging and discharging.
Such battery packs are often implemented with a battery monitoring device to maintain the cells in the battery pack within a safe operating state, which may include a protection circuit that limits the peak voltage of each battery cell during charge and prevents the cell voltage from dropping too low on discharge. Additionally, the temperatures of the cells may be monitored to prevent temperature extremes, since lithium-ion batteries are at risk of a self-initiated exothermal reaction, often referred to as thermal runaway. Overloading of a lithium-ion battery can set it on fire, which is not easily extinguished. During a thermal runaway, the high heat of the failing cell can propagate to the next cell, causing it to become thermally unstable as well. Thus, such a battery monitoring device may include protection circuitry to maintain the various cells' voltages and currents within safe limits so that the battery cells are in a safe operating state.
Rechargeable batteries are often configured as clusters or stacks of such batteries arranged into battery arrays, packs, or series-connected strings (collectively referred to herein as “battery packs”). A battery pack may include two or more cells coupled in series, with a number of series cell strings connected in parallel as an option. In multi-cell battery packs, problems may arise due to differences in the characteristics of the individual cells, i.e., cell capacities and/or cell charges. A difference in the cell ampere-hour capacities and/or cell charges may be referred to as cell imbalance. In a new battery pack, the cells generally start off well balanced. Cell imbalance may develop, however, due to aging or other causes, such as, for example, insertion of different grade cells due to repair, battery misuse, or differences in the active material quality of the cells at the time of cell manufacturing. Because of these and other problems resulting from cell imbalance, cell balancing techniques may be required to equalize the charge and/or capacity on every cell and/or prevent individual cells from becoming overstressed (i.e., functioning outside of a safe operating state).
This Background section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.